Method and apparatus for gasifying solid biomass fuel

ABSTRACT

A downdraft gasifier has a gas producer. The gas producer includes an introducing portion provided in an upper portion for introducing solid biomass fuel  4,  an air inlet  17  provided in a central portion, a fire grate  14  provided in a lower portion, and a product gas delivery portion provided below the fire grate. In this gasifier, there are provided fire-proofball laid layers in which a plurality of metal balls  15  are disposed in layers on the fire grate substantially uniform, and a preheating unit (such as a burner) for preheating the fire-proof balls. The metal balls  15  are preheated in advance to a predetermined temperature ranging from 500° C. to 800° C. before the solid biomass fuel is introduced into the producer. After that, the solid biomass fuel  4  is introduced into the producer, and gas produced in the producer is delivered from a portion below the fire grate.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method and apparatus forgasifying solid biomass fuel, and particularly relates to a gas producerand a gas producing method for use in a distributed power generatorusing solid biomass as fuel.

[0002] Biomass in which solar energy has been fixed and accumulatedin-vivo by plant photosynthesis is notable as an alternative powersource due to carbon or hydrogen contained therein.

[0003] There are some related-art power generation systems using biomassas fuel, such as 1) a system in which biomass fuel is burned directly ina boiler so as to generate power, for example, through a steam turbine;2) a system in which biomass fuel using microbes is fermented, andmethane gas extracted therefrom is, for example, fed to a fuel cell tothereby generate power; and 3) a system in which biomass fuel isgasified in a gas producer so as to produce combustible gas, and thecombustible gas is, for example, fed to a gas engine or a diesel engineso as to generate power.

[0004] When biomass is used as fuel for a boiler in the system 1),firewood, sawdust or chaff may be used directly as fuel. From the pointof view of efficiency, however, a power boiler of a medium size (500 kW)or larger is suited, and particularly this system is rarely used in adistributed power generator not higher than 300 kW. In addition, in thecharacter of the boiler, thermal inertia is so high that it takes longtime for starting and stopping. Therefore, long-term continuousoperation is desired in this system, and the system is not adequate to adistributed power generator started and stopped in a short time.

[0005] On the other hand, in the system 2) in which methane gas isproduced, the cost for equipment is high because large-scale equipmentsuch as a methane gas producer is required. Further, the cost fordisposal of a residue after the production of methane gas is alsorequired. Thus, there is a disadvantage that the total cost becomeshigh.

[0006] As the system 3) in which biomass fuel is gasified, there wasperformed, in 1983, a so-called sawdust power generation system in whichsawdust was gasified, and the product gas was fed to an engine through agas cleaner so as to generate power of several tens of kVA (seeCharcoal-Gas Car by Kozo Shionoya, 1996, POWERSHA Inc.,pp.17-18). Inthis case, there is a problem that the combustion efficiency is lowbecause sawdust is powdery.

[0007] In this case, it is considered that sawdust is formed into solidchips, such solid biomass fuel is gasified, and this gas is used forpower generation. By use of the solidified biomass fuel, it becomes easyto handle the fuel.

[0008] However, the solid biomass fuel is different from carbonized fuelsuch as charcoal in that a large amount of tar is contained in theproduct gas. There is a problem that such tar adheres to an intake valveor pipe arrangement of an engine so that long-term continuous operationbecomes difficult. To solve this problem, it is necessary to remove thetar by use of a special filter or a spray shower (see the above documentCharcoal-Gas Car, pp. 126-127). Alternatively, there is a method inwhich the gas producer is formed as a downdraft so as to burn and removetar.

[0009] As for such a downdraft gas producer, some patent applicationshave been proposed (for example, see International Patent PublicationNo. 2000-505123). A gas producer disclosed in the International PatentPublication No. 2000-505123 is apparatus for gasifying chiefly anorganic solid having a tendency to form slag. The same publication,however, discloses a downdraft gasifier in which wood or compost isformed into pieces, and the pieces are used as fuel. The gasifier has agas producer. The gas producer includes a fuel feed portion provided inan upper portion, an air inlet provided in a central portion, a firegrate provided in a lower portion, and a delivery portion provided underthe fire grate for delivering product gas. This gas producer is indeed adowndraft gas producer aimed at discharging the slag, but not aimed atburning and removing tar by downdraft. To burn and remove tar, producerstructure and operation based on a combustion mechanism are required.

[0010] Next, description will be made below about the details of themechanism of combustion in a gas producer (see the above documentCharcoal-Gas Car pp. 30-35) and a distributed power generation systemusing solid biomass as fuel.

[0011] Generally, the gasifying method is defined as a method in whichgas is produced chiefly by the reaction between carbon (C) and oxygen(O₂) or between carbon and steam at a high temperature. When only theair is sent into the producer, the following reactions are involved.

C+O₂═CO₂   (1)

C+(½)O₂═CO  (2)

CO+(½)O₂═CO₂  (3)

CO₂+C═2CO  (4)

[0012] Of these reactions, the reactions (1) and (3) are combustion oroxidation reactions, and exothermic reactions. On the other hand, thereaction (4) is a reduction reaction and an endothermic reaction.

[0013] This reduction reaction is also called a producer gas reaction.It is considered that carbon dioxide (CO₂) produced by the reaction (1)or (3) comes into contact with glowing carbon in a reduction layer sothat (CO) is produced by the reduction action. In a downdraft gasproducer, the air rich in oxygen comes into contact with carbonparticles so as to carry out an oxidation reaction. When the oxygen isconsumed and only carbon dioxide (CO₂) is left, the carbon dioxide comesinto contact with glowing carbon in a lower stage of the gas producer sothat (CO) is produced. Thus, in a fuel zone in the downdraft gasproducer, a reduction zone, an oxidation zone (or combustion zone) and adry distillation zone (or preheating zone) are generally formedsequentially in the order of increasing height from a fire grateprovided in the lower portion.

[0014] Next, description will be made on the outline of a distributedpower generation system using solid biomass as fuel, with reference toFIG. 5. FIG. 5 shows a block diagram of the schematic configuration ofthe system. In FIG. 5, a gasifier 50 includes a gas producer 51, a gascleaner 52 for cleaning product gas, and a gas cooler 53. Solid biomassfuel and the air are introduced into the gas producer 51.

[0015] Gas discharged from the gasifier 50 is fed to an engine 62through a gas mixer 61 for mixing the gas with the air, and used as fuelgas for the engine. Thus, electric power is generated by a powergenerator 63 coupled with the engine. Alternatively, the gas mixer 61may be provided integrally with the engine 62. In addition, the gascleaner 52 and the gas cooler 53 may be arranged integrally. Further,exhaust heat in the gasifier 50 or the engine 62 may be utilizedeffectively in accordance with necessity so as to form a co-generation(electricity and heat feed) system.

[0016] In the related-art downdraft gas producer, when gas containingtar passes through a high-temperature combustion zone again, there isindeed an advantage that the tar is burned and removed. However,problems arise as follows, on the other hand.

[0017] Among them, there is one problem that gas with a stable COconcentration ratio suitable as reduction gas cannot be obtained becausea combustion zone and a reduction zone are assumed to be mixed in thezone where the tar is burned, and it is therefore difficult to keep astable combustion state. Further detailed description on this problemwill be given later. In addition, there is another problem that the risetime required for the CO concentration to reach a high value suitable asengine gas is long because the CO concentration increases gradually whenthe gas producer is started.

SUMMARY OF THE INVENTION

[0018] The invention is achieved to solve the foregoing problems. Anobject of the invention is to provide a method and apparatus forgasifying solid biomass fuel, in which product gas contains no tar,starting and stopping of a gas producer are easy, gas having a stablecomposition can be obtained, and the rise time required for the stablecomposition is short.

[0019] To solve the foregoing problems, as in (1), the inventionprovides a downdraft gasifier for gasifying solid biomass fuel, having agas producer, the gas producer including a solid biomass fuelintroducing portion provided in an upper portion, an air inlet providedin a central portion, a fire grate provided in a lower portion, and adelivery portion provided below the fire grate for delivering productgas, wherein the downdraft gasifier further has: fire-proof ball laidlayers in which a plurality of fire-proof balls such as metal balls orceramics balls are disposed in layers on the fire grate substantiallyuniform; and a preheating unit for preheating the fire-proof balls.

[0020] By the gasifier, it is possible to form and keep a stablehigh-temperature reduction zone on the fire grate. Thus, as will bedescribed in detail later, the rise time required for the CO gasconcentration to reach a high level can be shortened, and the sameconcentration can be stabilized in the high level.

[0021] In addition, the invention stated in (2) and (3) are preferablefor carrying out the invention. That is, as in (2), the preheating unitis a detachable burner for preheating the fire-proof balls by combustionheat in the gasifier defined in (1). Thus, the preheating structure andthe preheating operation become simple and easy.

[0022] Further, as in (3), a cleaner/cooler for cleaning and cooling theproduct gas is provided in the gasifier defined in (1) or (2). Thus,high quality gas can be obtained. A cyclone or an oil filter can be usedas a cleaner. The oil filter may be integrated with a cooler.

[0023] On the other hand, the invention stated in (4) is preferable asthe method for producing gas. That is, as in (4), there is provided adowndraft gasifying method for gasifying solid biomass fuel, having thesteps of: introducing solid biomass fuel from an upper portion of aproducer; introducing combustion air from a central portion of theproducer; and delivering product gas from a portion below a fire grate,the fire grate being provided in a lower portion of the producer;wherein the downdraft gasifying method further has the steps of:preheating a plurality of fire-proof balls such as metal balls orceramics balls to a predetermined temperature in advance before thesolid biomass fuel is introduced, the fire-proof balls being disposed inlayers on the fire grate substantially uniform; and subsequentlydelivering gas from a portion below the fire grate, the gas beingproduced in the producer after the solid biomass fuel is introduced intothe producer. Thus, as described previously, an intended combustion andreduction operation can be obtained so that high quality product gas canbe obtained stably.

[0024] Further, the invention stated in (5) and (6) are preferable forcarrying out the invention. That is, as in (5), in the gasifying methoddefined in (4), the preheating temperature of the fire-proof ballspreheated ranges from 500° C. to 800° C. If the preheating temperatureis lower than 500° C., the operation and effect can be obtainedadequately. On the contrary, if the preheating temperature is higherthan 800° C., the operation and effect are indeed equivalent to thoseconducted at a temperature ranging from 500° C. to 800° C., butunnecessary preheating is unfavorable from the point of view ofreduction in total thermal efficiency.

[0025] In addition, as in (6), in the gasifying method defined in (4) or(5), after solid biomass fuel is introduced through a fuel feed gatefrom a solid biomass fuel feed hopper provided in an upper portion ofthe producer, the fuel feed gate is closed to stop fuel feed, and when afuel level in the producer reaches a predetermined level due to fuelconsumption in the producer, the fuel feed gate is opened to resume fuelfeed. Thus, it is possible to produce gas continuously and stably.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIGS. 1A and 1B are views showing a schematic configuration of agasifier according to an embodiment of the invention.

[0027]FIG. 2 is a view showing a schematic state when the gasifier shownin FIG. 1 starts.

[0028]FIG. 3 is a graph showing the result of an experiment of acombustion state according to the embodiment of the invention.

[0029]FIG. 4 is a graph showing the result of an experiment of acombustion state in a related-art gasifier.

[0030]FIG. 5 is a block diagram showing a schematic configuration of adistributed power generation system using solid biomass as fuel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] An embodiment of the invention will be described below withreference to the drawings.

[0032]FIGS. 1A and 1B are views showing a schematic configuration of agasifier for gasifying solid biomass fuel according to the invention.FIG. 1A shows a schematic state in a normal operation. FIG. 1B shows aview for explaining a rough configuration of a fuel feed gate 12 in FIG.1A. In addition, FIG. 2 shows a schematic state when the same apparatusis started. In FIGS. 1A, 1B and 2, the same constituent parts arereferenced correspondingly. The configuration of the gasifier will bedescribed below with reference to FIG. 1A.

[0033] In FIG. 1A, the gasifier includes a gas producer body 1, acyclone 2, a suction fan 3, and solid biomass fuel 4. Gas produced inthe gas producer body 1 is sucked by the suction fan 3. The sucked gaspasses through a gas delivery pipe 21, the cyclone 2 and a gas dischargepipe 22, and is fed to the engine side, for example, via another anot-shown oil-filter/cooler. In the cyclone 2, solid impurities areremoved by centrifugal action of the cyclone 2.

[0034] The gas producer body 1 includes a fuel feed hopper 11, a fuelfeed gate 12, a producer combustion portion 13, a fire grate 14, metalballs 15, a burner port 16, and an air inlet pipe 17. At the startingtime, a rod-like member of a burner simply illustrated as a member 18 inFIG. 2 is inserted into the burner port 16.

[0035] In this configuration, the metal balls 15 are disposed in layerssubstantially uniform. For example, balls of stainless steel or castiron can be used as the metal balls 15. In addition, the material of theballs is not limited to metal. These balls may be made of a fire-proofmaterial such as ceramics. The diameter of the balls is preferably in arange of about 30 mm to about 60 mm from the point of view of cloggingprevention or soaking effect.

[0036] Preferably, the fuel feed gate 12 is designed to be able to feedthe solid biomass fuel 4 from the fuel feed hopper 11 as uniformly aspossible with respect to the radial direction of the producer combustionportion 13. For example, the fuel feed gate 12 preferably has a rotarygate structure, as shown in FIG. 1B.

[0037] In FIG. 1B, the reference numeral 12 a represents a rotary gate;and 11 a, a hopper bottom disposed on the rotary gate. In the rotarygate 12 a, for example, a part of a rotary disc is notched at the angleof 300 centering the center of the disc, and a notch portion 12 b formedthus is used as a feed port for the solid biomass fuel 4. On the otherhand, a shield portion 11 b for covering the notch portion is providedin the hopper bottom 11 a.

[0038] In such a configuration, when the rotary gate 12 a is rotated,the notch portion 12 b moves sequentially in the circumferentialdirection. Thus, the solid biomass fuel 4 can be fed substantiallyuniformly. Incidentally, the fuel feed gate 12 comes into a closed modewhen the shield portion 11 b at the hopper bottom is superimposed on-thenotch portion 12 b of the rotary gate.

[0039] Next, description will be made below on the operation of the gasproducer body 1.

[0040] At the starting time before the solid biomass fuel 4 is charged,the metal balls 15 are preheated to 500-800° C. by the burner 18 asshown in FIG. 2. At this time, since the combustion exhaust gas of theburner is discharged from a portion below the fire grate 14 through gapsamong the metal balls, the metal balls are heated substantiallyuniformly. When the preheating of the metal balls 15 is completed, theburner 18 is extracted from the burner port 16, and the burner port 16is closed. Then, the solid biomass fuel 4 is charged. After the fuelcharge is completed, the fuel feed gate 12 is closed.

[0041] The fuel in the producer combustion portion 13 begins combustionin a zone above the preheated metal balls 15. Since the air inlet isprovided in the central portion of the producer combustion portion 13,the combustion zone (oxidation zone) moves upward gradually. As aresult, the zone right above the metal balls 15 becomes a reduction zonewhile a zone in the central portion near the air inlet becomes acombustion (oxidation) zone. A zone above the combustion zone (oxidationzone) becomes a preheated zone (dry distillation zone) Incidentally, tarcontained in gas is burned and reduced into combustible gas when it ispassing through the reduction zone.

[0042] When the solid biomass fuel in the producer combustion portion 13is consumed so that the fuel level is lowered, a not-shown level sensordetects that the fuel level has reached a predetermined level. On thebasis of this detection signal, the fuel feed gate 12 is opened to feedfuel again. For example, a known optical sensor can be used as the levelsensor. Alternatively, the fuel level may be detected on the basis ofthe correlation between the temperature change of the metal balls 15 andthe fuel level.

[0043] Next, description will be made below on an example of the resultof an experiment in which chaff chips were gasified into fuel gas by thesolid biomass fuel gasifier configured thus, in comparison with theresult of an experiment using related-art apparatus having no layershaving fire-proof balls such as metal balls laid.

[0044]FIG. 3 shows the result of the experiment according to theinvention when the preheating temperature of the metal balls (stainlessballs) is set at about 600° C. FIG. 4 shows the result of the experimentusing the related-art apparatus. In FIGS. 3 and 4, the abscissadesignates the lapsed time after combustion is started, and the ordinatedesignates the temperatures of an oxidation zone and a reduction zoneand the CO concentration (volume %) of product gas. Incidentally, inFIG. 3, the temperature of the metal balls (ball layer temperature) isalso shown.

[0045] First, description will be made with reference to FIG. 4. In therelated-art apparatus, at the starting time, heated charcoal is put on afire grate so as to start combustion. By the combustion start, thetemperature in the vicinity of an air inlet (oxidation zone temperature)increased to about 1,400° C. at a blast, and the temperature is keptsubstantially. The temperature of a reduction zone increased graduallyfrom about 400° C. The CO concentration increased gradually, but showeda low concentration value for about 1 hour after the point of time ofthe combustion start. After 1 hour had passed, the concentrationincreased comparatively. However, the CO concentration is not stable,but went up and down repeatedly.

[0046] This result can be considered as follows. That is, in the case ofthe related-art apparatus, the thickness of a combustion zone (includinga preheated zone, an oxidation zone and a reduction zone) is easy tochange in a small-scale producer, and the oxidation zone (combustionzone) and the reduction zone were mixed in the diameter direction of thesame section (where the producer is sectioned crosswise). Accordingly,it is difficult to keep a stable combustion state so that the COconcentration is not stable.

[0047] On the other hand, according to the result of the experiment ofthe invention in FIG. 3, the temperature of a reduction zone is higherat the start of combustion with ignition made by the metal balls. Whenabout 12 minutes had passed, the temperature of an oxidation zone andthe temperature of the reduction zone crossed each other and then becamereverse. In addition, at this point of time, the CO concentrationincreased suddenly and is stabilized. The CO concentration reached theceiling at 50%, because 50% is the highest scale in a measuringinstrument. Actually, the CO concentration not lower than the valueshown in FIG. 3 could be obtained. The CO concentration fluctuated inthe vicinity of the lapsed time of 30 minutes because fuel is chargedagain. Except the short time after the fuel is charged again, the COconcentration is stabilized again. There is no big change in the balllayer temperature. Incidentally, excluding CO, nitrogen (N₂) in the airis mainly contained in the product gas.

[0048] As is apparent from the comparison between FIG. 3 and FIG. 4, anideal combustion state is achieved by providing and preheatingmetal-ball laid layers and then burning solid biomass fuel. Thus, astable high-temperature reduction zone can be formed and kept on a firegrate so that the rise time required for the CO gas concentration toreach a high level can be shortened, and the same concentration can bestabilized in a high level.

[0049] As described above, according to the invention, a downdraftgasifier has a gas producer. The gasifier includes a solid biomass fuelintroducing portion provided in an upper portion, an air inlet providedin a central portion, a fire grate provided in a lower portion, and aproduct gas delivery portion provided below the fire grate. In thisgasifier, there are provided fire-proof ball laid layers in which aplurality of fire-proof balls such as metal balls or ceramics balls aredisposed in layers on the fire grate substantially uniform, and apreheating unit for preheating the fire-proof balls.

[0050] The fire-proof balls are preheated in advance to a predeterminedtemperature ranging from 500° C. to 800° C. before the solid biomassfuel is introduced. After that, the solid biomass fuel is introducedinto the producer, and gas produced in the producer is delivered from aportion below the fire grate.

[0051] Accordingly, when the solid biomass fuel is gasified, the productgas contains no tar, and starting and stopping of the gas producer areeasy. In addition, gas high in CO concentration and stable incomposition can be obtained. Further, the rise time required forreaching the stable composition can be shortened.

What is claimed is:
 1. A downdraft gasifier for gasifying solid biomassfuel, comprising: a gas producer including a solid biomass fuelintroducing portion provided in an upper portion, an air inlet providedin a central portion, a fire grate provided in a lower portion, adelivery portion provided below said fire grate for delivering productgas, fire-proof ball laid layers in which a plurality of fire-proofballs such as metal balls or ceramics balls are disposed in layers onsaid fire grate substantially uniform, and a preheating unit forpreheating said fire-proof balls.
 2. The gasifier for gasifying solidbiomass fuel according to claim 1, wherein said preheating unit is adetachable burner for preheating said fire-proof balls by combustionheat.
 3. The gasifier for gasifying solid biomass fuel according toclaim 1, further comprising: a cleaner/cooler for cleaning and coolingsaid product gas.
 4. A downdraft gasifying method for gasifying solidbiomass fuel, comprising the steps of: introducing solid biomass fuelfrom an upper portion of a producer; introducing combustion air from acentral portion of said producer; delivering product gas from a portionbelow a fire grate, said fire grate being provided in a lower portion ofsaid producer; preheating a plurality of fire-proof balls such as metalballs or ceramics balls to a predetermined temperature in advance beforesaid solid biomass fuel is introduced, said fire-proof balls beingdisposed in layers on said fire grate substantially uniform; andsubsequently delivering gas from a portion below said fire grate, saidgas being produced in said producer after said solid biomass fuel isintroduced into said producer.
 5. The method for gasifying solid biomassfuel according to claim 4, wherein said preheating temperature of saidfire-proof balls ranges from 500° C. to 800° C.
 6. The method forgasifying solid biomass fuel according to claim 4, wherein after solidbiomass fuel is introduced through a fuel feed gate from a solid biomassfuel feed hopper provided in an upper portion of said producer, saidfuel feed gate is closed to stop fuel feed, and when a fuel level insaid producer reaches a predetermined level due to fuel consumption insaid producer, said fuel feed gate is opened to resume fuel feed.